The capacitance graded bushings or condensive graded bushings are accessories applied in high and extra-high tension equipment, such as power transformers, derivation reactors, chain transformers and circuit breakers. Despite, in general, the cost of the referred graded bushings is not representative regarding the total price of the equipment to which is maid, a failure in its operation, as, for instance, an isolation failure, can elapse in serious damages, or even in the total loss of the equipment in which is maid.
The condensive graded bushings receive this denomination due to its constructive form, which origin a capacitance between your central conductor and the land. Once the graded bushing is energized, this capacitance allows the passage of a capacitive current to the land, advanced in 90° regarding the tension. Dielectric losses, generally measured through the tangent delta parameters or power factor, give origin to a resistive current, in phase with tension. The sum of these two components, capacitive and resistive, gives origin to the chain of escape of isolation.
Thus, any alteration in the two main parameters of isolation of the graded bushing, which are the capacitance and the dielectric losses, it causes a corresponding change in the escape chain (capacitive and resistive components respectively); what allows the monitoring systems, by means of the measuring of the escape chain, detecting in real time alterations in isolation of the graded bushing, be in the capacitance or in the tangent delta, avoiding thus catastrophic failures.
Associate to this aspect, other technical usually used in the processing of the measuring is the vectorial sum of the escape chains of a triphase group of graded bushings. This technical is based on the fact that any alteration in one of the escape chains of a joint triphase of graded bushings reflects equally in the summing. With that, it improves the measuring sensibility, become possible the same detection of small variations in isolation. With this technical, it is possible to measure only the alterations occurred in the capacitance and in the tangent delta, being necessary that the initial values of capacitance and tangent delta are programmed in the monitoring system, measured by off-line conventional techniques at the moment of the installation of monitoring system in real time.
Other characteristic of the vectorial sum technique of the escape chains are that, to make the referred sum, it is necessary that three graded bushings of a triphase system are connected to the monitoring system in real time and, when initiating its operation the system needs to pass by a “learning” period, during which the characteristics of graded bushings and of the electric system will be observed.